1. Field of the Invention
The present invention relates to acoustic sensing and alarm methods and devices for affixing to support components of a structure. More specifically, the invention relates to a self-managed sensing and alarm system for continuous monitoring of infrastructure structural integrity and deterioration, particularly with respect to assembled structures such as bridges and integrated structures such as bolted support roofs in underground mines.
2. Description of the Prior Art
According to the Mine Safety and Health Administration, 1,500-2,000 reportable roof falls occur each year in underground coal mines in the United States. Roof fall is the primary cause of accidental deaths in underground mines, resulting in dozens of deaths and thousands of injuries every year. According to MSHA reports, 70% of all accidental deaths in underground mines are caused by roof fall. The average cost of such an incident is between $1.5 million and $3.5 million, with an annual cost to the industry in the billions of dollars in lost production, repair, and cleanup.
There are 590,000 highway bridges in the United States. Many of these bridges are aging, posing catastrophic danger to human life in light of the undetected or underdetected impending failures or corrosion of structural components. The present state of the art for the assessment of structural health of bridges depends primarily on visual inspection that can recognize damage only in a late state of deterioration.
Referring to FIG. 1(a), Roof bolts 100 are typically placed four feet apart in order to create support roofs in underground mines. The roof bolts 100 are anchored in the roof 105, as shown in FIG. 1(b), utilizing a settable resin 110 around the bolt and are tightened to hold the various strata 115a, b of the roof 105 together. Less common is the use of an expandable mechanical anchor for anchorage. In either case, support is provided by the tensile load imparted to each bolt upon tightening the bolt head into tight abutment with the mine roof through a mounting plate 120 positioned between the mine roof 105 and the bolt head 125.
A number of prior art methods have been utilized to study and increase beam strength and improve roof support. Nevertheless, in time as mining continues, the strata 115 may start to separate and develop a tensile load on the bolt, as shown in FIG. 1(c). A layer of the strata may also shift horizontally exerting shear stress on the bolt. These forces can result in three different conditions causing roof collapse: (1) the bolt anchorage may fail causing the bolt to slide out of its position, (2) the bolt may fail and eventually break and (3) the roof may crack or separate above the layers held in place by the bolts, known in the art as a cutter roof failure. Statistically, the probability of roof falls in a given mine is 2.5 per year.
Past efforts to predict roof fall have not yielded viable results. A number of prior art references based on bolt loading, tension or strain measurements address the stress-strain relationship in materials. Generally, referring to FIG. 2, as the bolt is loaded or stressed, i.e., put under tension by weight of the overburden 130, typically clay, rock, coal or sand above the mine passage or tunnel, the bolt is strained, i.e., elongated. As shown in FIG. 2, when a critical stress and corresponding strain are reached, the bolt enters the yield region after which it breaks. One prior art approach was to measure the stress or strain, allowing impending bolt breakage to be detected. Experience in the field has shown that these methods are unreliable. Sometimes they predict bolt breakage when the bolt does not break, and at other times they do not predict bolt breakage when bolts do break.
The reason for this failure to predict bolt breakage is inherent in the variable monitored. Not all nominally identical bolts, i.e. identical model number, are in fact identical. The material from which a given batch of bolts is manufactured is not perfectly uniform. There will be certain variations from bolt to bolt. Manufacturing dimension tolerances compound these variations and the unpredictability. Consequently, the stress-strain curves for a given bolt model, in practice, display a spread as shown by area A in FIG. 2. Furthermore, the stress-strain curves for a given single bolt are different for different applied stress histories. In a mine roof, for example, if the bolt is stressed gradually over a long period of time, the curve will be different from a curve associated with spurts of stress over the same period of time, and still different from a curve associated with the same stress levels applied over a different period of time.
Most of the prior art is directed toward the measurement of load, strain or tension on the bolt, and several include the generation of a signal by the measuring device which is propagated within the bolt and the subsequent detection of changes to that signal over time, such as such as Spengler, et al., U.S. Pat. No. 4,149,446, issued Apr. 17, 1979 Popenoe, U.S. Pat. No. 4,114,428, issued Sep. 19, 1978; Choi, U.S. Pat. No. 4,318,302, issued Mar. 9, 1982 and Kibblewhite, U.S. Pat. No. 5,205,176, issued Apr. 27, 1993. Furthermore, the strain in a bolt in response to applied stress is different at different locations along the bolt. While the strain in a bolt is still in the safe zone at one location in the bolt, it may already be at the yield point at another location in the bolt. Installing multiple strain sensors on each bolt would make the system too expensive and measuring strain everywhere along the bolt is entirely impractical. Furthermore, strain data does not necessarily provide the needed information. Shear loading also contributes significantly to bolt failure in roof support in mines, which is not considered at all in load or strain measurements that use load cells, pressure sensitive discs or strain gages.
A different approach to predict roof fall addresses the measurement of roof sagging. Extensometers are used to determine the magnitude, position and rate of movement of soil or rock surrounding an excavation. They are widely used in mining to obtain support design information and as the basis of safety monitoring systems. Extensometers are installed into boreholes and, in mining, the smaller the diameter the better to minimize drilling costs. The simplest form of extensometer makes use of a stainless steel spring reference anchor with a tube indicator attached to it by stainless steel wire and visible at the hole mouth. Movement is indicated by colored reflective bands on the indicator, which are progressively covered as movement develops. In mining, a simple extensometer such as this is known as a “telltale” because it gives a visual indication of roof movement. A large number of such devices would have to be installed to cover a single mine. The National Institute of Occupational Safety and Health developed a Roof Monitoring Safety System that measures roof movement intended for use in wide-open roofs such as in room-and-pillar stone mines. NIOSH admits that this system is not suitable to predict roof fall.
Field use has shown that prediction based on telltale devices is unreliable. Roofs do collapse without prior indication from the device, and sometimes the device indicates an alert to an impending roof fall when the roof keeps staying intact. The failure of the telltale to forecast roof falls is rooted in the quantity that is being measured, i.e., roof sagging. The instrument measures by how much the roof at a given location has sagged relative to a reference point. The reference point is the anchorage location of the instrument that is assumed not to change, a questionable assumption, and furthermore, the instrument does not measure by how much the strata or the structure that holds the strata together has actually weakened.
Sagging of the mine roof results in vertical and horizontal stresses, imparting both axial and shear forces on the roof bolts. Combined tensile and shear forces are at times sufficiently large to cause bolt failure. Whether a bolt fails or not depends on the bolt material, structure and dimensions, on the anchorage resin, on the surrounding rock quality and on the angle between the bolt axis and the direction of the boundary between strata layers. None of these factors are considered or evaluated using the telltale instrument measurements. Therefore, the degree of roof sagging is not a measure of the structural state of the roof and the instrument does not reliably predict roof collapse. Although the strata may have shifted, the structure of the anchored bolts that support the strata may still be perfectly capable of holding the strata together. Alternatively, while roof lowering may be relatively small, the separation between the particular strata may have reached a critical value or the bolt structure that holds the roof together may have weakened to a critical level.
Other prior art methods of detection are based on studies of micro-seismic emission, which deploy geophones over mine roof areas. The geophones upper frequency limit ranges between 4.5 and 14 Hz. High frequencies on the order of hundreds or thousands of kilohertz cannot be detected over large areas because of severe attenuation of high frequency pressure or sound waves. Such systems require the installation of geophones in boreholes in mines and moving or adding them into new boreholes as mining advances. Under this system, in order to determine whether a roof fall is imminent, and its location, it is necessary to combine four computed parameters and apply human interpretation. To date these seismographic studies are unable to reliably predict roof fall. Two problems associated with this approach are that high frequencies cannot be detected and that location determination depends on the speeds of sound wave propagation in various directions. These speeds are not reliably predictable, as they depend on the rock strata's non-homogeneous structures.
In summary, while the need to be able to predict impending roof fall in underground mines or other structural failure in rigid support structures, such as bridges, in time to be able to take proactive action to prevent failure and related injuries is lacking in the art. A system is necessary to identify an alarm condition in time to take proactive action to prevent failure.